John Roberts imagined himself as a consensus-builder after his confirmation to be the 17th chief justice of the Supreme Court, a justice in the mold of John Marshall charged with alleviating divisions on the Court by advocating judicial modesty. Some progressive observers took these claims very seriously. I was inclined to view them as essentially fraudulent.

Well, score one for the optimists.

Today, the Supreme Court upheld the Patient Protection and Affordable Care Act in its entirety. While this outcome was not shocking, the vote lineup could be to many observers given that the final vote was 5-4. Chief Justice John Roberts joined the Court's four liberal members to uphold the PPACA, while frequent swing voter Justice Anthony Kennedy joined a remarkably radical dissent.

The ultimate effects of NFIB v. Sebulius remain uncertain, but at the very least the case compels a re-evaluation of both Roberts and Kennedy. While Chief Justice Roberts deserves substantial praise for not striking down the signature policy of an incumbent administration based on constitutional arguments that crumble upon the slightest inspection, this praise should not be excessive. Roberts's idiosyncratic reasons for upholding the PPACA are less than ideal and may place unnecessary limits on future government power.

Thanks to the Chief Justice, the Supreme Court's holding today is somewhat compromised and ungainly, like the PPACA itself. But also like the PPACA, it's a major improvement over the viable alternative. In the case of health-care reform, this alternative was an appalling status quo; when it comes to today's ruling, it was the de facto repeal of the New Deal constitutional order.

The Court's four Democratic appointees, represented by Ruth Bader Ginsburg's brilliant, powerful concurrence, would have correctly upheld the PPACA as a valid exercise of the federal government's interstate commerce and spending powers. Chief Justice Roberts upheld the PPACA for a different reason—not as a valid exercise of the federal power to regulate interstate commerce but as a valid exercise of the federal taxing power.

"The Affordable Care Act’s requirement that certain individuals pay a financial penalty for not obtaining health insurance may reasonably be characterized as a tax," concluded Chief Justice Roberts. "Because the Constitution permits such a tax, it is not our role to forbid it, or to pass upon its wisdom or fairness."

It is hard to ignore that by upholding the PPACA on this basis, Chief Justice Roberts has handed a political weapon to the Republicans, who can portray the PPACA as a major tax increase. Another potentially disturbing element of Chief Justice Roberts's opinion is the limitations he places on the federal spending power. Roberts—joined on this point by Justices Kagan and Breyer—held that the limitations placed on the states by the federal government's expansion of Medicaid went too far. The federal government could place conditions on the new Medicaid grants, Roberts argued, but could not threaten existing Medicaid funding without being unconstitutionally coercive. This part of Roberts's opinion is unfortunate, making it more likely that a few red states will reject the Medicaid expansion for no compelling reason. Still, because Roberts's opinion permits the federal government to place conditions on the new Medicaid funds created by the PPACA (if not the existing ones), the fundamental federal spending power remains in place, and it's not clear that the distinction will have any serious impact going forward. The same cannot be said of the dissent, which proposes a radical revision of federal powers.

Lest there be any doubt about how seriously the four Republican dissenters take their proposed constitutional revolution, they departed from the usual practice of joining an opinion written by one justice and co-signed the opinion jointly. This kind of joint opinion is generally reserved for a handful of historically monumental cases in which the Court wants to make a statement about its authority (such as the school desegregation case Cooper v. Aaron and the decision to uphold Roe v. Wade in Planned Parenthood v. Casey.) This was no ordinary dissent, and the fact that it was fully joined by Justice Kennedy is a major statement.

Kennedy and his three fellow dissenters were not advocating a minimalist opinion, such as striking down the mandate but leaving the rest of the bill intact. Instead, they concluded that they "would find the [PPACA] invalid in its entirety." Not only the individual mandate but the many other important aspects of the bill would have been ruled unconstitutional. The fact that the justices found that the mandate could not be severed from the rest of the act underscores the radicalism of their agenda. It is well settled that Congress has the power to pass laws that are "necessary and proper" to the exercise of its enumerated powers. This creates obvious problems for the dissenters, who simultaneously argue that the individual mandate is not sufficiently related to interstate commerce and that it is so integral to the rest of the bill that it must be struck down as a whole. This only makes sense if one assumes that—contrary to their protests elsewhere—the dissenters think that any major health-care reform exceeds federal power.

While Chief Justice Roberts's arguments about the commerce clause were limited to the narrow facts of the mandate and would not have any clear effect on future cases, the vision of the dissenters reflects a far more limited vision of federal power. Even more radical is Kennedy and the other dissenters' view that the Medicaid expansion in general (and not just the denial of existing Medicare funds) exceeds the federal spending power. The conditions placed on the grant of Medicaid money, the dissenters argue, are unconstitutionally coercive: "The offer of the Medicaid Expansion was one that Congress understood no State could refuse." The problem is that this would leave the existing constitutional order a complete shambles, as the same could be said of any other exercise of the federal spending power—including Medicare and federal education spending—which involve deals that it is impractical for states to turn down. Even more than the arbitrary restrictions on the power to regulate interstate commerce, this would represent a serious threat to the Great Society programs most Americans cherish.

What this decision makes particularly clear is the stark choice facing Americans in November. In a sense, it is futile to speculate how Roberts's hair-splitting will play out, because with four members of the Court over the age of 70, this will be determined by a different bench. Thanks to Chief Justice Roberts, the constitutional order represented by the New Deal and Great Society remains intact—but there are now four justices on record as rejecting it. If Mitt Romney is charged with replacing Ruth Bader Ginsburg, the federal powers that have been taken for granted since 1938 will almost certainly perish. The constitutional revolution being proposed by Justices Kennedy, Scalia, Alito, and Thomas did not succeed today—but it is frighteningly close, and should it be realized, the effects on America's most vulnerable citizens would be catastrophic.